Add uma stats to help evaluate the impact of changes to the quota allocation logic.

storage/common/storage_histograms.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef STORAGE_COMMON_HISTOGRAMS_H
+#define STORAGE_COMMON_HISTOGRAMS_H
+
+#include <string>
+#include "storage/common/storage_common_export.h"
+
+namespace storage {
+
+STORAGE_COMMON_EXPORT void RecordBytesWritten(const char* label, int bytes);
+STORAGE_COMMON_EXPORT void RecordBytesRead(const char* label, int bytes);
+
+}  // namespace storage
+
+#endif  // STORAGE_COMMON_HISTOGRAMS_H